D&R Comp - A Workalike for either DynaComp or Ross Compressors The lazy, sustain-for-days sound of a lot of rock guitar sounds was only partially the result of the guitar construction and/or amp settings. In many cases, the guitarist was using a compressor, a kind of volume controller that varied its internal gain to keep the sound through it at about the same level no matter what the actual signal level was. The original Dynacomp compressor, built by MXR, was a very popular means of achieving this sound. The D-comp uses the same technology as the original Dynacomp to get the same tone. A later variant of the same circuitry was the Ross Compressor. This effect varied from the Dynacomp by using a bit of additional bypassing Shown with True Bypass switching for the Millenium Bypass. Connect this point to 9V and filtering. Otherwise, they are the conrol pad of the MB to get an LED indicator with the same DPDT switch. In identical. It's possible to make a Out R15 single board to do both - this is the R13 D&R Comp. C4 Q1-5 = high gain
D-Comp Compressor
Vb
R2
R16
C13 D2
R19 R14
U1 6 3 5 4
C9
Q2 Q4
Q5 Q3
SUST R20 LEVEL
R17
C15
D3
C8
R12
C10
R21
R8
R7
C14 R5
R1
Electro Cap Polarity
2 7
C11
TRIM1
R3
Q1C6
R18
Vb
D1
R10
Vb
C1
Parts not in the original but included for modern pedal practices.
C16
R11
Copyright 1999 R.G. Keen All rights reserved. Fair use only if this notice is reproduced. No permission to serve from web pages other than http://www.geofex.com/
9V
In
R-Comp Compressor
Shown with True Bypass switching for the Millenium Bypass. Connect this point to the conrol pad of the MB to get an LED indicator with the same DPDT switch.
Out R13
U1 6
4 TRIM 1
R11
Copyright 1999 R.G. Keen All rights reserved. Fair use only if this notice is reproduced. No permission to serve from web pages other than http://www.geofex.com/
R18
R16 R17
C15
R21
R12
C8
D3
R7
R8
R5
Q3
C14 C10
C9
Q4
Q5
5
C6 Not in original pedal
Q2
C16
SUST R20 LEVEL
3
Q1
R19
2 7
C13 D2
R6
Vb Vb C11
R3
C3
R14
R10 C7
C5
C4
R9
C12
Vb
R4
D1
R1
C1
R15 C2
How it works: Input capacitor C1 isolates the effect's internal DC bias level from the 0Vdc level of the guitar. In the D-Comp, Q1 is biased to a reference DC level by resistor R3 to the Vb bias voltage source created by the R13/R15 divider. In the R-comp, R4 is added in series with R3 and the junction bypassed to ground by capacitor C4. Q1 supplies a lowimpedance buffered signal at its emitter. This buffered signal is routed to the inverting input (pin2) of the OTA. The noninverting input (pin 3) of the OTA is held at the same DC level as pin 2. The OTA is biased by R10 to the +9V supply and R7/R8 to ground at the two ends of the 2K bias-balance trimmer . The trimmer serves to balance the
NPN silicon, such as 2N5088; will work with 2N3904 or 2N4124. Diodes 1N914 or 1N4148.
R2
The heart of the D&Rcomp is a variable gain amplifier built around a Operational Transconductance Amplifier or OTA. The CA3080 OTA IC (U1 on the schematic) has a differential input, and a gain that is dependent on the amount of bias current supplied to its pin 5, which sets the bias current of the device, and its gain. The rest of the circuit is housekeeping, to make the OTA happy in its role of providing variable gain, or the level detecting circuit to supply that proper bias current.
D-Comp and R-Comp Compressors
input currents of the differential inputs and balance out any control voltage noises as the gain of the OTA changes. In the R-Comp, R6 is added in series with R10, and the junction of the two are bypassed to ground by C5. The maximum gain of the OTA is set by the R14 load resistor attached from pin 6 to the bias voltage, and the high frequency rolloff is set by the parallel combination of R14 and C11. From pin 6, the signal goes to the base of Q2. Q2 performs two tasks - it acts as an output buffer to drive the output signal, and also as a phase inverter to help derive the DC level signal which controls the gain of the whole circuit. The output signal is tapped from the emitter of Q2, while both the emitter and collector of Q2 provide out-of-phase signals to a rectifier-filter arrangement built from Q3 and Q4. The most negative peak of the both phases of the signal from Q2 is clamped to ground by the diodes at the bases of Q3 and Q4, providing a "rectified" signal that is as large as the peak-to-peak Q2 signal. Since there are two of these working on the two out-of-phase signals from Q2, the signal is effectively full wave rectified. Q3 and Q4 invert; that is, higher signals cause them to pull their collectors to a lower voltage. The collectors pull this current from R18 to +9V, pulling down on the voltage of C16. In the absence of a signal, C16's voltage rises to nearly the supply voltage. When large signals are passed through, Q3 and Q4 pull the voltage on capacitor C16 lower. The voltage on this capacitor is buffered by Q5, and in turn drives the bias current of the OTA through the 500K sensitivity control and the 27K resistor. So as a result, if there is a very small or no signal at the input, Q3 and Q4 do not pull down on C16, and the voltage at the emitter of Q5 is high; this supplies a current to the bias pin which is determined by the voltage at Q5's emitter and the Sustain control in series with R12. With a high voltage at Q5's emitter, the bias voltage into Pin 5 of the OTA is high, and the gain of the OTA is high. As the signal level rises, Q3 and Q4 pull the voltage on the 10uF capacitor down, so the voltage across Sustain and R12 are lower, and current is reduced to the bias pin so the OTA gain goes down. Note that the sustain control can vary the current over a wide range as it varies from 0 to 500K. This setup effectively forms a negative feedback loop which attempts to adjust the signal level at the output of Q2 so it is almost constant. When a note is first hit, it is loud and the level feedback adjusts the OTA to a low gain. As the note trails off, the feedback circuit lets the gain come up to hold the output signal almost constant until the maximum gain of the circuit is reached and the OTA can no longer keep the tiny signal up to the desired level. The stomp switch on the vintage units is a DPDT switch, but not wired for true bypass. The guitar always connected to the input of the effect unit. Because the input signal must go through a 10K resistor at the input to the effect, the loading may be bad enough to cause "tone sucking". I have drawn the circuit with true bypass switching to get around this. I have also shown how to wire the units for true bypass with an LED status indictor; see the "Millenium Bypass" at GEO, http://www.geofex.com for how to build the circuit to do that. Building it: While neither circuit is complicated, and the use of a printed circuit board (PCB) does make things much easier, you will need to pay attention to the differences between the D-comp and R-comp versions. In the Parts List that follows, I have shaded the entries where the D-comp and R-comp versions are different. The R-comp version uses more parts, but the D-comp version needs jumpers strategically placed. It's probably a good idea to study the differences in the two population diagrams before beginning building, or in fact before ordering your parts, just to be sure that you get the right set of parts. I like to build things in a specific order: •
clean the copper side of the board to ensure easy soldering. Acetone, kitchen scrubbing powder or just 0000 steel wool all do a good job
•
insert and solder in any jumpers; the jumpers on this board all replace resistors, so they are on the same 0.4" spacing as the resistors.
•
insert resistors; all the resistor holes are on 0.4" centers, so you could make a piece of wood or plastic that's the right width to bend many of them to the right spacing at the same time; I also use a lead bender gadget that I got at Mouser. Very, very handy.
•
Bend the resistor leads slightly to hold them in the board as you insert them, and clip them off, leaving about 1/16" of lead protruding above the copper.
•
Solder in the resistors.
•
Place the capacitors next, clenching the leads slightly and then clipping and soldering. 2 Copyright 1999-2001 R.G. Keen. All rights reserved.
D-Comp and R-Comp Compressors •
Once all the passive parts are in place, double check the values and correct any placement mistakes. I often mistake 4.7K for 47K, as I confuse the red with the orange bands.
•
Double check the pinouts of the transistors, then insert them and solder them into position
•
Finally, solder the IC in. I like to tack-solder pins 4 and 8 to get the IC placed and flat on the PCB, and once it's all in position, solder the rest of the pins.
You're probably wondering if there is any difference in the sound of the D-comp and R-comp versions. There is. It's not big, but there is a slight advantage in smoothness of compression in favor of the R-comp.
Notes on the parts list: [1.] The original transistor type number is not know. 2N5088's worked well in the prototype. You can probably get away with 2N3904 and 2N4124 as well. [2.] The D &R Comp PCB is designed to fit into the Hammond 1590BB die cast aluminum box. This is a sturdy unit, and with care in fitting parts into it, you can make a pedal which will stand up to the rigors of road gigs. [3.] Parts different between the two versions are shown shaded gray in the parts list. Be sure to check and buy the right parts for the version you want. [4] Part numbers and prices listed were current when this was written. Since part numbers and prices change all the time, do not be surprised if some of them have changes. Just ask the sales rep to help you find a replacement part that they do have.
3 Copyright 1999-2001 R.G. Keen. All rights reserved.
D-Comp and R-Comp Compressors
Parts List Part
D-Comp
R-Comp
Description
Mouser Stock No.
Est. $$$
U1 Q1-Q5 D1-D3 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 Sust Level
CA3080 2N5088 1N914/4148 4.7M 10K 1M jumper 10K jumper 1M 1M jumper 470K 15K 27K 56K 150K 27K 1M 1M 150K 10K 10K 10K 0.01uF not used 1uF electro not used not used 1uF electro not used 1uF electro 10uF electro 0.01uF 0.01uF optional 0.01uF film 0.01uF film 0.047uF film 10uF electro 500K lin pot 50K log pot
CA3080 2N5088 1N914/4148 4.7M 10K 470K 470K 10K 220K 1M 1M 10K 220K 15K 27K 56K 150K 27K 1M 1M 150K 10K 10K 10K 0.01uF 0.0022uF 1uF electro 1uF electro 1uF electro 1uF electro 1uF electro 1uF electro 10uF electro 0.01uF 0.01uF 0.1uF cer. 0.01uF film 0.01uF film 0.047uF film 10uF electro 500K lin pot 50K log pot
Operational Transconductance Amplifer High gain, low noise NPN transistor Signal diode ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor ¼ W carbon film resistor Film capacitor Film capacitor Radial package aluminum electrolytic cap Radial package aluminum electrolytic cap Radial package aluminum electrolytic cap Radial package aluminum electrolytic cap Radial package aluminum electrolytic cap Radial package aluminum electrolytic cap Radial package aluminum electrolytic cap Film capacitor Film capacitor Ceramic capacitor Film capacitor Film capacitor Film capacitor Radial package aluminum electrolytic cap Potentiometer Potentiometer
570-CA3080E 610-2N5088 625-1N914 29SJ250-4.7M 29SJ250-10K 29SJ250-1M or 470K 29SJ250-470K 29SJ250-10K 29SJ250-22K 29SJ250-1M 29SJ250-1M 29SJ250-10K 29SJ250-470K or 220K 29SJ250-15K 29SJ250-27K 29SJ250-56K 29SJ250-150K 29SJ250-27K 29SJ250-1M 29SJ250-1M 29SJ250-150K 29SJ250-10K 29SJ250-10K 29SJ250-10K 140-PF1H103K 140-PF2A222K 140-XRL25V1.0 140-XRL25V1.0 140-XRL25V1.0 140-XRL25V1.0 140-XRL25V1.0 140-XRL25V1.0 140-XRL25V10 140-PF1H103K 140-PF1H103K 21RZ310 140-PF1H103K 140-PF1H103K 140-PF1H473K 140-XRL25V10 313-1000-500K 313-4000-50K
0.68 0.26 0.06 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.07 0.11 0.10 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.11 0.11 0.08 0.11 0.11 0.17 0.05 1.21 1.74
Part
Description
enclosure stomp switch knobs batt clip
Hammond 1590BB or eq. DPDT stomp switch. Preferably Carling 317PP to your own taste. PP3 / 9V battery clip
Mouser Stock No. Est. $$$ Not stocked by Mouser
4 Copyright 1999-2001 R.G. Keen. All rights reserved.
D-Comp and R-Comp Compressors
5 Copyright 1999-2001 R.G. Keen. All rights reserved.
